Running GLib Applications

Running GLib Applications — How to run and debug your GLib application

Running and debugging GLib Applications

Environment variables

GLib inspects a few of environment variables in addition to standard variables like LANG, PATH or HOME.

G_FILENAME_ENCODING. This environment variable can be set to a comma-separated list of character set names. GLib assumes that filenames are encoded in the first character set from that list rather than in UTF-8. The special token "@locale" can be used to specify the character set for the current locale.

G_BROKEN_FILENAMES. If this environment variable is set, GLib assumes that filenames are in the locale encoding rather than in UTF-8. G_FILENAME_ENCODING takes priority over G_BROKEN_FILENAMES.

G_MESSAGES_PREFIXED. A list of log levels for which messages should be prefixed by the program name and PID of the application. The default is to prefix everything except G_LOG_LEVEL_MESSAGE and G_LOG_LEVEL_INFO.

G_DEBUG. If GLib has been configured with --enable-debug=yes, this variable can be set to a list of debug options, which cause GLib to print out different types of debugging information.

fatal_warnings	Causes GLib to abort the program at the first call to g_warning() or g_critical(). This option is special in that it doesn't require GLib to be configured with debugging support.
fatal_criticals	Causes GLib to abort the program at the first call to g_critical(). This option is special in that it doesn't require GLib to be configured with debugging support.
gc-friendly	Newly allocated memory that isn't directly initialized, as well as memory being freed will be reset to 0. The point here is to allow memory checkers and similar programs that use bohem GC alike algorithms to produce more accurate results. This option is special in that it doesn't require GLib to be configured with debugging support.
resident-modules	All modules loaded by GModule will be made resident. This can be useful for tracking memory leaks in modules which are later unloaded; but it can also hide bugs where code is accessed after the module would have normally been unloaded. This option is special in that it doesn't require GLib to be configured with debugging support.
bind-now-modules	All modules loaded by GModule will bind their symbols at load time, even when the code uses %G_MODULE_BIND_LAZY. This option is special in that it doesn't require GLib to be configured with debugging support.

The special value all can be used to turn on all debug options. The special value help can be used to print all available options.

G_SLICE. This environment variable allows reconfiguration of the GSlice memory allocator.

always-malloc

This will cause all slices allocated through g_slice_alloc() and released by g_slice_free1() to be actually allocated via direct calls to g_malloc() and g_free(). This is most useful for memory checkers and similar programs that use Bohem GC alike algorithms to produce more accurate results. It can also be in conjunction with debugging features of the system's malloc implementation such as glibc's MALLOC_CHECK_=2 to debug erroneous slice allocation code, allthough debug-blocks usually is a better suited debugging tool.

debug-blocks

Using this option (present since GLib-2.13) engages extra code which performs sanity checks on the released memory slices. Invalid slice adresses or slice sizes will be reported and lead to a program halt. This option is for debugging scenarios. In particular, client packages sporting their own test suite should always enable this option when running tests. Global slice validation is ensured by storing size and address information for each allocated chunk, and maintaining a global hash table of that data. That way, multi-thread scalability is given up, and memory consumption is increased. However, the resulting code usually performs acceptably well, possibly better than with comparable memory checking carried out using external tools. An example of a memory corruption scenario that cannot be reproduced with G_SLICE=always-malloc, but will be caught by G_SLICE=debug-blocks is as follows:

		    void *slist = g_slist_alloc(); /* void* gives up type-safety */
		    g_list_free (slist);           /* corruption: sizeof (GSList) != sizeof (GList) */

The special value all can be used to turn on all options. The special value help can be used to print all available options.

G_RANDOM_VERSION. If this environment variable is set to '2.0', the outdated pseudo-random number seeding and generation algorithms from GLib-2.0 are used instead of the new better ones. Use the GLib-2.0 algorithms only if you have sequences of numbers generated with Glib-2.0 that you need to reproduce exactly.

LIBCHARSET_ALIAS_DIR. Allows to specify a nonstandard location for the charset.aliases file that is used by the character set conversion routines. The default location is the libdir specified at compilation time.

TZDIR. Allows to specify a nonstandard location for the timezone data files that are used by the #GDateTime API. The default location is under /usr/share/zoneinfo. For more information, also look at the tzset manual page.

Locale

A number of interfaces in GLib depend on the current locale in which an application is running. Therefore, most GLib-using applications should call setlocale (LC_ALL, "") to set up the current locale.

On Windows, in a C program there are several locale concepts that not necessarily are synchronized. On one hand, there is the system default ANSI code-page, which determines what encoding is used for file names handled by the C library's functions and the Win32 API. (We are talking about the "narrow" functions here that take character pointers, not the "wide" ones.)

On the other hand, there is the C library's current locale. The character set (code-page) used by that is not necessarily the same as the system default ANSI code-page. Strings in this character set are returned by functions like strftime().

Traps and traces

Some code portions contain trap variables that can be set during debugging time if GLib has been configured with --enable-debug=yes. Such traps lead to immediate code halts to examine the current program state and backtrace.

Currently, the following trap variables exist:

static volatile gulong g_trap_free_size;
static volatile gulong g_trap_realloc_size;
static volatile gulong g_trap_malloc_size;

If set to a size > 0, g_free(), g_realloc() and g_malloc() will be intercepted if the size matches the size of the corresponding memory block. This will only work with g_mem_set_vtable (glib_mem_profiler_table) upon startup though, because memory profiling is required to match on the memory block sizes.

Note that many modern debuggers support conditional breakpoints, which achieve pretty much the same. E.g. in gdb, you can do

break g_malloc
condition 1 n_bytes == 20

to break only on g_malloc() calls where the size of the allocated memory block is 20.

Gdb debugging macros

glib ships with a set of python macros for the gdb debugger. These includes pretty printers for lists, hashtables and gobject types. It also has a backtrace filter that makes backtraces with signal emissions easier to read.

To use this you need a recent enough gdb that supports python scripting. Gdb 7.0 should be recent enough, but branches of the "archer" gdb tree as used in Fedora 11 and Fedora 12 should work too. You then need to install glib in the same prefix as gdb so that the python gdb autoloaded files get installed in the right place for gdb to pick up.

General pretty printing should just happen without having to do anything special. To get the signal emission filtered backtrace you must use the "new-backtrace" command instead of the standard one.

There is also a new command called gforeach that can be used to apply a command on each item in a list. E.g. you can do

gforeach i in some_list_variable: print *(GtkWidget *)l

Which would print the contents of each widget in a list of widgets.

SystemTap

SystemTap is a dynamic whole-system analysis toolkit. GLib ships with a file glib.stp which defines a set of probe points, which you can hook into with custom SystemTap scripts. See the files glib.stp and gobject.stp which are in your shared SystemTap scripts directory.

Memory statistics

g_mem_profile() will output a summary g_malloc() memory usage, if memory profiling has been enabled by calling g_mem_set_vtable (glib_mem_profiler_table) upon startup.

If GLib has been configured with --enable-debug=yes, then g_slice_debug_tree_statistics() can be called in a debugger to output details about the memory usage of the slice allocator.